1. Field of the Invention
The present invention relates to a process for producing low molecular weight isoolefin polymers from a higher molecular weight saturated isoolefin polymer.
2. Description of Information Disclosures
Processes for producing a lower molecular weight isoolefin polymer from a higher molecular weight saturated isoolefin polymer are known and include solution processes, in which the reaction is carried out in an appropriate solvent as the reaction medium and bulk or melt phase processes, in which the reaction is carried out with the polymer itself as the reaction medium. Because of the high viscosity of high molecular weight polymers, solution processes have been the focus of much academic work [e.g., L. M. Pyrkov and S. Ya. Frenkel, Russian Chemical Reviews, 32, 140 (1963)]. From an industrial point of view, solution processes have several disadvantages.
Firstly, the polymer must be dissolved. Polymer dissolution rates are typically low and the solubility of the polymer in the best solvents may also be limited, leading to long dissolution times (several hours) and large vessels to contain the dilute polymer solution. No such vessels are required in a bulk phase process. Secondly, as the polymer is diluted its reaction rate with any reagent is generally reduced, except for the unusual case of a reaction which has a zero order rate dependence on polymer concentration. Longer reaction times or higher reaction temperatures are needed to cope with the lower reaction rates, in addition to the necessarily larger size of the reaction vessels compared with those for a bulk phase process. Thirdly, the presence of a solvent presents the possibility of side reactions, not possible when the solvent is not present. Such side reactions not only may reduce desired reaction yields, but may give side products, which may present environmental health and safety problems. This is of particular concern as solvents are typically volatile and their reaction products may easily escape into the atmosphere. Fourthly, the polymer product must be recovered from solution. Again, additional vessels and time are required, compared to a bulk phase process. The recovery of polymers from solution is often the major component of the cost of production for polymers prepared by solution processes and this last point may often be the most significant debit for a solution process.
Because of these disadvantages of solution processes, a bulk or melt phase process for a polymer reaction is highly desirable.
The terms "bulk phase" and "melt phase" are used herein interchangeably.
Bulk phase processes for producing a lower molecular weight isoolefin polymer, such as isobutylene polymer, from a higher molecular weight saturated isoolefin polymer, such as a higher molecular weight isobutylene, in the absence of a free radical initiator by thermal or mechanical means are known. The product produced by such processes is often discolored.
U.S. Pat. No. 3,634,381 discloses a process and apparatus for the degradation of high molecular weight polyisobutylene in a specified screw extruder at specified conditions. See also U.K. patent 1,293,671.
European patent application, Publication No. 0,035,677 A 1, discloses a process for the degradation of high molecular weight polyisobutylene at a temperature of 150.degree. to 400.degree. C. in the presence of tocopherol as heat stabilizer to prevent carbon deposition during degradation.
European patent application, Publication No. 0 111 391 A 2, discloses a process for reducing the molecular weight of unsaturated isoolefin polymers, such as butyl rubber, and halobutyl rubber, wherein the unsaturated polymer is masticated, in the melt phase, in the presence of ozone at a temperature of 20.degree. to 22.degree. C. The process is performed in an internal mixer, such as a Banbury mixer or an extruder.
U.S. Pat. No. 4,614,772 discloses a process for producing a low molecular weight isobutylene-conjugated diene copolymer, such as butyl rubber, by feeding an unsaturated higher molecular weight isobutylene-conjugated diolefin copolymer, such as an isobutylene-isoprene copolymer, into an extruder and contacting the unsaturated higher molecular weight copolymer, with an oxygen-containing gas, such as air, in the absence of an antioxidant and in the absence of an added free radical initiator.
European patent application 8530778.2, Publication No. 0 180 446 A2, discloses a process for the production of a low molecular weight isobutylene-conjugated diene diolefin polymer by reacting an unsaturated isobutylene-conjugated diolefin polymer of higher molecular weight with an oxygen-containing gas, such as air, in a specified twin screw extruder at specified conditions.
The term "saturated" is used herein to denote that the polymer is made by the polymerization or copolymerization of only mono-olefinic monomers (e.g., isobutylene, styrene, para-methylstyrene). The term "unsaturated" is used herein to denote that the polymer is made by the copolymerization of a mixture containing at least one diolefinic monomer (e.g., isoprene) and that for every molecule of the diolefinic monomer incorporated into the copolymer one olefinic double bond is produced in the copolymer.
U.S. Pat. No. 3,862,265 discloses a process for modifying the rheological or chemical properties of a polymer which is normally solid at room temperature, in an extruder. A free radical may be introduced into a reaction zone of the extruder. The process is stated to be suitable for modifying all classes of elastomers, including natural rubber, polyisobutylene, butyl rubber, etc. (See Col. 4, lines 42 to 48). It discloses that many polymers, particularly polyolefins, can be suitably narrowed in their molecular weight distribution by this process (Col. 5, lines 26 to 32).
It has now been found that a low molecular weight isoolefin polymer can be produced from a higher molecular weight saturated isoolefin starting material, in the melt phase, in the presence of a molecular oxygen-containing gas or a specified free radical initiator to produce a low molecular weight polymer having a molecular weight distribution substantially the same or broader than that of the high molecular weight saturated polymer starting material.